EXCRETION AND DISTRIBUTION OF AMMONIA AND THE INFLUENCE OF BOUNDARY LAYER ACIDIFICATION IN EMBRYONIC RAINBOW TROUT (ONCORHYNCHUS MYKISS)

摘要

This study examined ammonia excretion by embryos of the rainbow trout (Oncorhynchus mykiss). The distribution of ammonia in relation to the H~+ distribution and electrical potential was determined. The influence of the pH of the unstirred layer (USL)of water next to the external surface of the embryo was also assessed. Eyed-up embryos (35-40 days post-fertilization) were exposed to various external water conditions [pH6.0. pH10.0, 1.6mmol l~(-1) NaCl, 0.0mmol l~(-1)Cl, 0.2 mmol l~(-1) NH_4Cl, 2.5mmol l~(-1) borax buffer (Na_2B_4O_7-10H_2O), 2.5 mmol l~(-1) Hepes, O.lmmol l~(-1) amiloride] for 30min and ammonia excretion rates, ammonia concentration in the perivitclline fluid (PVF) and yolk, and the pH of the PVF, yolk and USL were measured. The rate of ammonia excretion was dependent, in part, on the partial pressure gradient of NH_3 (△P_(NH_3) from the PVF to the USL. Exposure to water of pH6 increased, whereas NH_4Cl or pH 10 exposure decreased, ammonia excretion rates. Elevated external Na~+ levels also influenced the rate of ammonia excretion, but neither Na~+-free water nor amiloride had any effect. The distribution of ammonia between the PVF and USL was dependent on the H~+ distribution, but ammonia Was distributed according to the electrical potential between the PVF and yolk. The USL was 0.32pH units more acidic than the bulk water. Addition of buffer to the external water eliminated the acid USL and decreased ammonia excretion rates. We conclude that rainbow trout embryos excrete ammonia primarily as NH_3, but when external Na~+ levels are elevated, ammonia excretion may be independent of the P_(NH_3) gradient. The acidic USL next to the chorion probably facilitates NH_3 diffusion by maintaining the △P_(NH_3) through the conversion of NH_3 to NH_4~+ upon entry into the USL.